You're standing at the rental car counter at the airport. You're exhausted and all you want is to get your car and head to your hotel -- and when the agent asks if you want to buy insurance on your rental, your shoulders sag even more. You're afraid to decline, but you've also heard it's a waste of money. And that's a pretty big chunk of cash you'd be spending: Insurance can run you up to $40 extra per day, depending upon the coverage.

For years, savvy travelers declined the rental car insurance. They figured their regular auto insurance would cover them. Today, the question remains: Do you need rental car insurance? Like the answer to many questions about the subject, the answer is "it depends." Well, what determines your need for rental insurance? Let's look at the variables.

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First, consider the insurance you already have. How well does your regular auto policy cover use of a rental car? Many policies cover rental cars, but read the fine print on your policy or call your agent ahead of time before you hit the road.

Some people save on their regular auto policies by buying the minimum required auto insurance and/or by having high deductibles. Most states require car owners to have liabilityinsurance, which applies if your car causes harm to someone else or damages that person's property. You can add collisioncoverage, which will help pay to fix your car if you hit another car or object, or if something hits your car. Comprehensivecoverage pays to repair or replace your car in something other than a collision -- if it's stolen, for example, or ruined in a flood. Even if you choose to go without collision and comprehensive coverage on your own car, or have a high deductible (the amount you pay before insurance kicks in), you may not want to take that chance on an expensive rental car. If that's the case, you might want the rental car insurance.

Numbers also play a role. If you plan to drive the rental car with someone who's not on your regular insurance, make sure that person has his or her own coverage before declining rental car insurance.

Speaking of numbers, watch out for nickel-and-diming. Rental companies often charge extra fees that your regular policy won't cover -- for loss of use of the vehicle while it's in the shop, depreciation and "administrative" costs. You may want to buy the rental company's collision damage waiver (an agreement that you won't have to pay for damage to the car in most circumstances) if your credit card won't cover them. However, many credit cards provide secondary coverage that takes care of fees and charges that your regular auto policy won't cover. Keep in mind that this applies only if you pay for the rental entirely with that credit card and decline the rental company's coverage. It won't cover damages if they were caused by a driver who isn't on the rental contract. What's more, the cards usually won't pay anything until you've exhausted your usual insurance, and they won't cover certain things, like loss of use, without a fight. It may be in your best interest to get a card that prioritizes rental car insurance coverage.

If you don't own a car and don't have regular auto insurance, you may be required to buy some rental car insurance, depending on laws in the state where you drive. Even if you're not required to buy coverage, it's probably a good idea unless you're prepared to cover all the costs of an accident. For those who don't have regular policies, or whose policies don't cover rental cars, most rental car companies offer coverages for personal injury and death of the driver and passengers, damage to personal property and additional coverage for injury or damage to others. Like we said before, it depends.

Author's Note

I've actually texted my sister (who travels more than I do) to reassure myself that I didn't need to buy the rental company's insurance – while I was standing in line to pick up my rental car. I always have a "what if" moment of doubt when I decline the coverage, so I was really interested in researching this question.

I was reassured to learn that my regular auto policy, like most, does extend coverage's to cars that I rent. I didn't know about the extra fees the rental companies often tack on if you do find yourself in an accident. But by far the biggest surprise was learning that my credit card gives me some protection to cover what my regular insurance doesn't, as long as I charge the rental on that card. Who knew that credit cards give you something you didn't even know to ask for? And once again, I was reminded of the importance of doing some research before you find yourself at that moment of decision.